1. Field of the Invention
The present invention relates to fire-resisting constructions, and more particularly refers to a fire door structure having improved means provided for framing a fire-resistant core and facing skins.
2. Description of the Prior Art
For years the various building codes have accepted a one and three-quarter inch thick solid core wood door, or a solid wood core door, as an appropriate method of protecting an opening between a room and a corridor. Such doors performed acceptably in many fires where they provided necessary protection between a room and a corridor in an individual house, a hotel guest room and a corridor, an apartment house dwelling unit and a corridor, hospital or nursing home patient rooms and a corridor, and so on. However, in the last twenty years or so, the increasing awareness of both a need for and the provision of increasingly more fire resistant measures and also the increasing scarcity and cost of acceptable wooden door cores, and the necessary labor and skill to solidly build them and provide effective fire barriers, has lead to an increased strictness of building codes. With modern high-rise construction becoming more prevalent, it has become even more necessary today than ever before to use fire doors of that type which will exhibit good fire-resistance so that they will pass the requisite fire tests and will withstand the conditions to which they are subjected during ordinary day-to-day usage and during intense fires in such buildings. Responsive to these needs, there has been considerable development of various door core materials as the insulation buffer between the outer skins and considerable development in the techniques of constructing fire doors with such newer core materials. There have even been some changes in the outer skins or facings of the doors. However during all this time the choices for suitable stiles and rails to form the framing of such doors have remained at only wood, sometimes impregnated with a fire retarding salt chemical, and metal.
Wood, of course remains the standard framing material of fire door construction. But it is the weakest link to contain a fire, especially with increased duration or intensity, or both, of the fire. In standard fire tests ordinary wood framing burns out in about one half an hour; and while chemically impregnated wood lasts longer it still fails in less than an hour. The problem is not only the smoldering and then igniting of the wood but also the ensuing structural failure. If the wood around the hinges or latch burns away, then the door may fall. Also the merely burning away of an area creates sufficient opening for the passage of smoke and eventually the passage of the fire itself around the door barrier. Currently tests for fire ratings subject doors to a fire to test for both burning and smoke passage and then apply a fire hose stream of water to test for structural integrity. Generally wooden framed doors burn through and collapse in just over a half hour and generally less than one hour even when impregnated with a fire retardant chemical salt during the fire portion of the test.
Metal rails and stiles suffer certain disadvantages also. Firstly various metals are quite heat conductive; and when the metal facing skin on one side of the door is exposed to the heat of a fire it readily transmits its heat to the metal framing members, and they quickly transmit the heat onto the opposite side of the door and its facing skin. Increasingly, building codes are recognizing that the heat build-up on the side of the door away from a fire can cause ignition to nearby objects such as curtains, draperies and other room furnishings etc. In addition, as the temperature of most metals rises, the metal expands and this expansion causes a warping, twisting and distortion. When metal framed fire doors begin to warp, with the intense heat of high temperature fire tests and increased duration of the fire test, the entire door structure bends allowing openings to occur and thus allowing the passage of smoke and flame. This causes the door to fail the fire test.
Another material which has been used to a great extent in the construction of panels for non-structural uses has been gypsum or plaster and it has been proposed to adhere metal facing sheets to such panels. For example U.S. Pat No. 3,866,376 provides a demountable wall assembly of a metal clad gypsum panel in a hollow wall assembly. Such an arrangement is in no way suggested for, or contemplates, a fire door assembly as disclosed herein. Also, the advantage of combining gypsum with small amounts of fibrous materials such as glass fibers to extract some of the good properties of each was early recognized; for example, see U.S. Pat. No. 1,719,726 issued on July 2, 1929 to Raynes which disclosed a gypsum wallboard, one or more of the edges of the body of which are provided with a strip of fibrous material which might for example be impregnated with gypsum. In that instance, the board was provided with a loose fibrous material i.e. short chopped fibers in dispersed random arrangement to increase the resistance of the edges to chipping and cracking during handling, shipping or the like. Such short random fibers provided only a capacity to absorb surface shock of the fragile material, while most if not all of the tensile strength of the panel was provided by the gypsum material and very little if any bending strength or ductility was provided to the panel. U.S. Pat. No. 3,616,173 is to much the same effect, as is U.S. Pat. No. 2,744,022. Further, U.S. Pat. No. 3,311,516 also relates to gypsum sheets for building constructions but utilizing large quantities of a very fine and small asbestos fiber to form their sheets so such would be without appreciable bending strength; and U.S. Pat. Nos. 2,892,339 and 3,106,503 teach the making of honey comb structural panels with inter-connected gypsum "cells." While such products are widely utilized as finishing materials in the covering of room walls, corridors, shaft walls, demountable partitions and the like, they do not possess sufficient strength, particularly bending strength to serve as the framing member of structural panels and doors. This material is not particularly resistant to bending forces such as those created when doors are repeatedly subjected to the types of stress normally applied to them. That is, upon repeated openings and closings, along with intermittent slamming to which a door is subjected in ordinary usage, and particularly where it is a thick generally much heavier door, the material is just too brittle to withstand the constant impact and would shatter and break in usage without being subjected to a fire. In addition in the area of door construction there is a current trend toward finding inexpensive doors meeting a minimum fire rating. That is doors which will consistently and inexpensively meet a 20 minute fire rating. Here either the cores or facing sheets are not very critical on fire resistance but the framework in addition to fire resistance must possess sufficient bending strength if a material substitution for wood or metal is to be found.